1. Field of the Invention
The present invention relates to a motor driving circuit, and, more particularly, to a motor driving circuit for single-phase full-wave driving a single-phase brushless DC motor (hereinafter, simply referred to as a motor).
2. Description of the Related Art
In the case of driving a motor in a single-phase full-wave mode, an H-bridge circuit configuration is generally used. However, when a commutation occurs at a motor coil in an H-bridge circuit, transistors arranged at an upper arm and a lower arm in the H-bridge circuit may be short-circuited for an extremely short time interval at the times of turn-on and turn-off of each transistor, thereby causing a through current. For preventing such a through current, a generally employed method is to introduce a dead time at each of the times of the turn-on and turn-off of each transistor.
FIG. 10 is a circuit diagram showing a conventional motor driving circuit, which is disclosed in, e.g., Japanese Patent Application Publication No. 2005-269855.
In FIG. 10, the reference character 1 designates a Hall element for detecting positions of magnetic poles in a magnet. Further, a comparator circuit includes a first comparator 2 for converting an output voltage into a first square wave signal; and a second comparator 3 for producing a second square wave signal that corresponds to an inverse voltage of an output of the first comparator 2. Additionally, ZD refers to a Zener diode, R11-R21 refer to resistors, C11 and C12 refer to capacitors, and D11 and D12 refer to diodes.
Further, a dead time circuit includes a first dead time circuit having resistor R16 and capacitor C12 configured to smooth a rising edge in a voltage waveform of the first square wave signal; and a second dead time circuit having resistor R17 and capacitor C11 configured to smooth a rising edge in a voltage waveform of the second square wave signal.
In addition, an H-bridge circuit includes a lower arm having a first transistor Tr1 and a second transistor Tr2 which are MOSFET transistors; and an upper arm having a third transistor Tr3 and a fourth transistor Tr4 which are PNP transistors, and a motor coil 6.
FIGS. 11A to 11F are timing charts describing operations of the conventional motor driving circuit. More specifically, FIG. 11A shows an output voltage of the Hall element 1; FIG. 11B shows the first square wave signal obtained as the output voltage of the first comparator; FIG. 11C shows an output voltage of the first dead time circuit; FIG. 11D shows an output voltage of the second dead time circuit; FIG. 11E shows an ON/OFF signal of the first transistor Tr1; and FIG. 11F shows an ON/OFF signal of the second transistor Tr2.
By using the dead time circuits including resistors and capacitors, the rising times of the voltage waveforms of the first and the second square wave signal are increased as respectively shown in FIGS. 11C and 11D. Further, due to the influence of the cutoff voltage between the gate and the source of each of the transistors Tr1 and Tr2 on the turn-on and turn-off operations thereof, dead times Td1 and Td2 are secured while the transistors Tr1 and Tr2 are being turned on, respectively, as shown in FIGS. 11E and 11F. The dead times prevent the through currents at the respective transistors in the H-bridge circuit.
Further, as a method for enhancing the motor efficiency, it has been commonly proposed that the electric conduction is prohibited during an initial stage and a final stage of a half-period (equivalent to an electric angle of 180°) in a counter-electromotive force waveform, and is allowed during the time the counter electromotive force reaches a predetermined level. In this regard, Japanese Patent No. 3239054, for example, discloses a method of controlling a conduction angle in the case of half-wave driving.
As discussed above, in the conventional motor driving circuit, the dead time circuit has to be provided to prevent the through current that may flow through the motor coil during the time of a commutation. This increases the number of electric components, which in turn results in a cost increase. Further, also in case of driving the motor by setting a conduction angle for enhancing the motor efficiency, a relatively large number of electric components are required, thereby complicating the circuit.